STEREO Hunts for Remains of an Ancient Planet near Earth

April
9, 2009: NASA's twin STEREO probes are entering a
mysterious region of space to look for remains of an ancient
planet which once orbited the Sun not far from Earth. If they
find anything, it could solve a major puzzle--the origin of
the Moon.

"The
name of the planet is Theia," says Mike Kaiser, STEREO
project scientist at the Goddard Space Flight Center. "It's
a hypothetical world. We've never actually seen it, but some
researchers believe it existed 4.5 billion years agoâand that
it collided with Earth to form the Moon."

Right:
An artist's concept of one of the STEREO spacecraft. [Larger
image]

The
"Theia hypothesis" is a brainchild of Princeton
theorists Edward Belbruno and Richard Gott. It starts with
the popular Great Impact theory of the Moon's origin. Many
astronomers hold that in the formative years of the solar
system, a Mars-sized protoplanet crashed into Earth. Debris
from the collision, a mixture of material from both bodies,
spun out into Earth orbit and coalesced into the Moon. This
scenario explains many aspects of lunar geology including
the size of the Moon's core and the density and isotopic composition
of moon rocks.

It's
a good theory, but it leaves one awkward question unanswered:
Where did the enormous protoplanet come from?

Belbruno
and Gott believe it came from a Sun-Earth Lagrange point.

Sun-Earth
Lagrange points are regions of space where the pull of the
Sun and Earth combine to form a "gravitational well."
The flotsam of space tends to gather there much as water gathers
at the bottom of a well on Earth. 18th-century mathematician
Josef Lagrange proved that there are five such wells in the
Sun-Earth system: L1, L2, L3, L4 and L5 located as shown in
the diagram below.

When
the solar system was young, Lagrange points were populated
mainly by planetesimals, the asteroid-sized building blocks
of planets. Belbruno and Gott suggest that in one of the Lagrange
points, L4 or L5, the planetesimals assembled themselves into
Theia, nicknamed after the mythological Greek Titan who gave
birth to the Moon goddess Selene.

Above:
Sun-Earth Lagrange points. The STEREO probes are about to
pass through L4 and L5. Solar observatories often park themselves
at L1 while deep space observatories prefer L2. [more]

"Their
computer models show that Theia could have grown large enough
to produce the Moon if it formed in the L4 or L5 regions,
where the balance of forces allowed enough material to accumulate,"
says Kaiser. "Later, Theia would have been nudged out
of L4 or L5 by the increasing gravity of other developing
planets like Venus and sent on a collision course with Earth."

If
this idea is correct, Theia itself is long gone, but some
of the ancient planetesimals that failed to join Theia may
still be lingering at L4 or L5.

"The
STEREO probes are entering these regions of space now,"
says Kaiser. "This puts us in a good position to search
for Theia's asteroid-sized leftovers."

Just call them "Theiasteroids."

Astronomers
have looked for Theiasteroids before using telescopes on Earth,
and found nothing, but their results only rule out kilometer-sized
objects. By actually entering L4 and L5, STEREO will be able
to hunt for much smaller bodies at relatively close range.

Right:
This dynamical simulation shows how asteroids linger in the
gravitational well of a Lagrange point of the Sun-Jupiter
system. The principle of Sun-Earth Lagrange points is the
same. Credit: Prof. Aldo Vitagliano/SOLEX.

"The
search actually began last month when both spacecraft rolled
180 degrees so that they could take a series of 2-hour exposures
of the general L4/L5 areas. In the first sets of images, amateur
astronomers found some known asteroids and new comet Itagaki
was imaged just a couple of days after the announcement of
its discovery. No Theiasteroids however."

Hunting
for Theiasteroids is not STEREO's primary mission, he points
out. "STEREO is a solar observatory. The two probes are
flanking the sun on opposite sides to gain a 3D view of solar
activity. We just happen to be passing through the L4 and
L5 Lagrange points en route. This is purely bonus science."

"We
might not see anything," he continues, "but if we
discover lots of asteroids around L4 or L5, it could lead
to a mission to analyze the composition of these asteroids
in detail. If that mission discovers the asteroids have the
same composition as the Earth and Moon, it will support Belbruno
and Gott's version of the giant impact theory."

The
search will continue for many months to come. Lagrange points
are not infinitesimal points in space; they are broad regions
50 million kilometers wide. The STEREO probes are only in
the outskirts now. Closest approach to the bottoms of the
gravitational wells comes in Sept-Oct. 2009. "We have
a lot of observing ahead of us," notes Kaiser.

Readers,
you may be able to help. The STEREO team is inviting the public
to participate in the search by scrutinizing photos as they
come in from the spacecraft. If you see a dot of light moving
with respect to the stars, you may have found a Theiasteroid.
Links to the data and further instructions may be found at
sungrazer.nrl.navy.mil.